Rotary fluid film face seals, also called gap or non-contacting face seals, are usually applied to high-speed and/or high-pressure rotating equipment wherein the use of ordinary mechanical face seals with face contact would result in excessive heat generation and wear. Non-contacting operation avoids this undesirable face contact at times when the shaft is rotating above a certain minimum speed, which is called a lift-off speed.
There are various ways of accomplishing the above non-contacting operation. One of the more commonly used ways includes the formation of a shallow spiral groove pattern in one of the sealing faces. The sealing face opposite the grooved face is relatively flat and smooth. The face area where these two sealing faces define a sealing clearance is called the sealing interface.
The above-mentioned spiral groove pattern on one of the sealing faces normally extends inward from the outer circumference and ends at a particular face diameter called the groove diameter, which is larger then the inner diameter of the seal interface. The non-grooved area between the groove diameter and the inner interface diameter serves as a restriction to fluid outflow. Fluid delivered by the spiral pattern must pass through this restriction and it can do so only if the sealing faces separate. The way this works is through pressure build-up. Should the faces remain in contact, fluid will be compressed just ahead of the restriction, thus building up pressure. The pressure causes separation force which eventually becomes larger than the forces that hold the faces together. In that moment the sealing faces separate and allow the fluid to escape. During operation of the seal, an equilibrium establishes itself between fluid inflow through spiral pumping and fluid outflow through face separation. Face separation is therefore present as long as the seal is operating, which means as long as one face is rotating in relation to the opposite face.
However, spiral pumping is not the only factor that determines the amount of the separation between the sealing faces. Just as the spirals are able to drive the fluid into the non-groove portion of the sealing interface past the groove diameter, so can the pressure differential. If enough of a pressure difference exists between the grooved end of the interface and the non-grooved end, fluid will also be forced into the non-grooved portion of the interface, thereby separating the faces and forming the clearance.
Both ways in which clearance can be formed between the sealing faces, one with speed of rotation, the other with pressure differential, are distinct and separate, even though the effects of both combine on the operating seal. If there is no pressure difference and the seal face separation occurs strictly due to face rotation, forces due to fluid flow are known as hydrodynamic forces if the fluid sealed is a liquid, and aerodynamic forces if the fluid sealed is a gas.
On the other hand, if there is no mutual rotation between the two sealing faces and face separation is strictly the consequence of pressure differential between both ends of the sealing interface, forces due to fluid flow are called hydrostatic forces if the fluid sealed is a liquid, and aerostatics forces if the fluid sealed is a gas. In the following, the terms hydrostatic and hydrodynamic are used for both liquid and gas effects since these latter terms are more conventionally used when describing both liquid and gas seals.
A typical spiral groove seal needs to provide acceptable performance in terms of leakage and the absence of face contact during all regimes of seal operation. It must do so not only at top speed and pressure, but also at standstill, at start-up, acceleration, at periods of equipment warm-up or at shutdown. At normal operating conditions, pressure and speed vary constantly, which results in continuous adjustments to the running clearance. These adjustments are automatic; one of the key properties of spiral groove seals is their self-adjustment capability. On change in speed or pressure, the face clearance adjusts automatically to a new set of conditions. Hydrostatic and hydrodynamic forces cause this adjustment.
The operating envelope of speeds and pressures is usually very wide and a seal design of necessity must be a compromise. For its performance to be acceptable at near-zero speed or pressure, it is less than optimum at operating speed and pressure. This is simply due to the fact that, both in terms of pressure and speed, the seal has to be brought up to operating conditions from zero speed and zero pressure differential.
Especially critical to seal operation is the start-up. If the seal is applied to a centrifugal gas compressor, the full suction pressure differential is often imposed onto the seal before the shaft starts turning. This presents a danger in that the sealing faces will lock together with friction. Face lock results when the hydrostatic force is insufficient to counter pressure forces that maintain the seal faces in contact. Face lock can lead to seal destruction, in which excessive break-away friction between contacting seal faces can cause heavy wear or breakage of internal seal components.
First then, spiral grooves must be able to separate the sealing faces hydrodynamically for full speed non-contacting operation. This normally requires fairly short and relatively deep spiral grooves. Second, the spiral grooves must be able to unload the sealing faces hydrostatically for start/stops to prevent face lock. For this, the grooves have to be extended in length. The extended grooves in turn cause more separation and leakage during full speed operation. The full speed leakage of a typical 3.75 inch shaft seal with short and relatively deep spirals may be about 0.9 SCFM (i.e. Standard Cubic Feet per Minute) at 1,000 psig and 10,000 rpm. However, full speed leakage for such a seal with extended grooves may reach 2.4 SCFM under the same conditions, almost triple the previous value. The constant burden of larger-than-necessary leakage represent significant operating costs and is highly undesirable.
Spiral groove design practice goes back to U.S. Pat. No. 3,109,658 wherein two opposing spiral grooves pump oil against each other to develop a liquid barrier capable of sealing a gas. Such an arrangement is limited in pressure as well as speed capability, as is inherent in the use of liquid forces to seal gas.
Another known arrangement is shown in U.S. Pat. No. 3,499,653. This interface design with partial spiral grooves relies heavily on hydrostatic effects. The interface gap is designed with a tapered shape which is narrower at the non-grooved end and wider at the spiral grooves. The effect of the spiral grooves and therefore the hydrodynamic forces are suppressed since spiral groove pumping becomes less effective across the wider gaps. This likewise affects the stability of the seal and limits its top pressure and speed capability.
A further known arrangement is shown by U.S. Pat. No. 4,212,475. Here the spiral groove itself attempts to act both as a hydrostatic as well as a hydrodynamic pattern and is used to eliminate the need for the tapered shape of the gap so that a considerable degree of spiral groove hydrodynamic force can be applied to impart a self-aligning property to the sealing interface. The self-aligning property forces the sealing interface back towards a parallel position, regardless of whether deviations from parallel position during seal operation occur in radial or tangential directions. This resulted in improvement stability and increased performance limits in terms of pressure and speed.
While the known fluid seals as briefly summarized above have attempted to provide both hydrodynamic and hydrostatic sealing properties, nevertheless the known seals have been deficient with respect to their ability to optimize the combination of these hydrostatic and hydrodynamic properties so as to provide desirable hydrostatic properties which facilitate starting and stopping of seals while effectively minimizing or avoiding direct face contact and minimizing face loading between the seals so that the assembly can be started up with minimal friction to avoid severe frictional power requirements and direct frictional wear between the faces, and at the same time provide desirable hydrodynamic properties between the relatively-rotatable seal faces under a wide range of operating conditions particularly those involving high speed and high pressure.
Accordingly, it is an object of this invention to provide an improved fluid seal of the type employing a grooved pattern on one of the opposed seal faces, which improved seal provides a more optimized combination of hydrodynamic and hydrostatic sealing characteristics so as to permit improved seal performance under a significantly greater range of operating conditions, including operating conditions ranging from start-up to conditions involving high speed and high pressure.
In the improved seal arrangement of the present invention, the groove pattern (which is typically defined on only one of the seal faces) includes first and second groove arrangements both of which communicate with the high pressure fluid at one side of the seal, one groove arrangement being significantly deeper than the other, whereby the deeper arrangement is particularly effective for providing the desired hydrodynamic characteristics, whereas the shallower groove arrangement primarily provides only hydrostatic characteristics. At the same time, these arrangements are positioned such that the shallower arrangement is interposed generally radially between the deeper groove arrangement and a non-grooved annular land or dam which effectively separates the groove pattern from the low pressure side of the seal, whereby desirable hydrostatic and hydrodynamic seal properties can both be obtained but at the same time leakage of sealing fluid (for example, a gas) across the dam to the low pressure side is minimized so as to improve the performance efficiency of the seal.
In the improved seal of this invention, as briefly discussed above, the groove pattern includes the deep groove arrangement which is defined by a circumferentially arranged series of grooves which angle circumferentially and radially inwardly from the surrounding high-pressure side of the seal, which angled grooves may be of spiral, circular or straight configuration. These angled grooves are relatively deep and project only partway across the seal face. In the preferred embodiment, the radially inner ends of the angled grooves communicate with the shallow groove arrangement which is positioned radially inwardly of the deep groove arrangement, but which is separated from the low pressure side of the seal by the intermediate non-grooved annular land or dam. This shallow groove arrangement has a depth which is a small fraction of the depth of the deeper groove arrangement and is effective for creating a hydrostatic force between the opposed sealing faces substantially in the central region thereof as defined between the radially outer and inner boundaries of the seal interface.
In addition, this optimization of the seal properties and performance characteristics is further improved by optimizing the groove pattern or configuration relative to the surrounding lands defined on the seal face so that the fluid film which is created between the opposed seal faces provides a more uniform pressure distribution and sealing characteristics while minimizing distortion of the seal face, which in turn assists in optimizing the seal performance with minimum width of gap between the opposed seal faces while still avoiding or minimizing direct contact and frictional wear between the opposed seal faces.
Further improvement to the seal arrangement is aimed at reduction and elimination of seal face distortions that normally occur as a result of circumferential non-uniformity of hydrostatic pressure fields as these form above groove and land regions at conditions at or near to the zero speed of rotation. This improvement is achieved in the shallow groove arrangement by a narrow and shallow circumferential groove disposed radially adjacent the radially inner ends of the angled grooves. The shallow circumferential groove is in continuous pressure fluid communication with the angled grooves and acts to equalize hydrostatic pressure field non-uniformities circumferentially, as a result suppressing any face distortions and producing a uniform face separation with no or only minimal face-to-face contact even at extremely low magnitudes of separation between the faces.
Other objects and purposes of the invention will be apparent to persons familiar with seals of this general type upon reading the following specification and inspecting the accompanying drawings.